Anish Roshi & Frank Ghigo (Jan 25, 2002)
This note describes Radio Frequency Interference (RFI) measurements in the 385--520 MHz frequency band of the GBT to determine which frequency ranges are relatively free of RFI. The survey was done with the PF1 450 MHz receiver system of the GBT. The time variability of the RFI was also examined by repeating observations over a 8 hour period. The feed arm servo contribute significantly to the RFI in this band and therefore we recommend to turn it `off' during observations. There are several RFI in this band which are time variable. More than 15 % of the band will be RFI free even when the servo system is ``on''. With the servo system `off', the RFI free frequency range in the spectrum with 120 sec integration can be as high as 40 % or more. Many frequency ranges (especially with servo system `off') within this band can be used for astronomical observations. With the servo system turned `off', continuum observations in spectroscopic mode could use more than 40 % of any 40 MHz frequency band within the PF1 450 MHz band. 15 to 18 out of 27 galactic recombination lines are located at frequency ranges relatively free of RFI when the servo system is turned "off".
A data set for monitoring RFI in the 385 to 520 MHz was obtained on Jan 23-24, 2002 using the PF1 450 MHz receiver of the GBT in dual polarization mode. The backend used was the spectral processor in 1024 channel, 40 MHz bandwidth mode. To scan the full frequency band of the 450 MHz system using the spectral processor, the center frequencies were set to 400, 435, 470 and 505 MHz for successive data acquisitions. The reference frame was set to topocentric for these tests. Spectra from two orthogonal linear polarizations were collected using 2 FFT banks of the spectral processor in the above described mode. A data scan (ie spectrum with a center frequency) consists of four 30 secs integrations thus forming a total integration of 120 secs. The data was collected at an interval of about 0.5 hr starting at about Jan 23, 23:00 hours and ending at Jan 24, 8:00 hours. The antenna was pointing at elevation 75 deg and azumuth 185 deg during the survey with the ``antenna tracking system on''.
Fig. 1 shows the 120 secs averaged raw spectrum (top) from one of the polarizations and band-shape corrected spectrum (bottom) at the time (in UT) indicated on the title. A running five point median filtered raw spectrum was used as an estimate of the band-shape. The raw spectrum is useful to identify any broad RFI feature which will not be present in the band-shape corrected spectrum. The yellow line in the bottom figure can be used as a guide to identify the RFI components picked up by an automatic RFI detection glish routine. The glish routine computes a spectral RMS after eliminating most of the `strong' RFI from the band-shape corrected spectrum using an iterative method. All spectral values above this RMS are considered as RFI. Note that this RMS is still larger than the RMS in the spectrum with no RFI. The frequencies of the RFI components thus picked are written into the file rfi400jan24.freq . The solid vertical lines in the bottom figure show 300 KHz spectral windows (50 KHz below the rest frequency of hydrogen and 250 KHz above it) near the rest frequencies of hydrogen recombination line transition.
Fig. 2 shows 26 minutes averaged band-shape corrected spectra from one polarization. The 300 KHz spectral windows near hydrogen recombination lines are shown (light-blue line) in this plot as in Fig. 1. All the spectra shown in Fig. 1 corresponding to each frequency range are averaged to get this plot. From Fig. 2, it is clear that about 15 % of a 40 MHz frequency range could be used for continuum observations (see spectra 4 to 8 of Fig 1 for data with servo system "off"). The frequencies and relative amplitudes of the RFI components picked up by the automatic RFI detection glish routine are written into the file avrg400jan24.freq . As in Fig.1, the yellow line can be used as a guide to identify the RFI components picked up by the glish routine.
Fig. 3 shows 120 sec integrated band-shape corrected spectra from one linear polarization when the feed arm servo system is turned ``on'' (light-blue) and ``off'' (yellow). Significant `strong' RFI from feed arm servo is observed.
Fig. 5 shows the gray scale display of the 13 spectra on each 40 MHz band taken over 8 hours. Several RFI features are time variable.
The feed arm servo contribute significantly to the RFI in this band and therefore we recommend to turn it `off' during observations. There are several RFI in this band which are time variable. More than 15 % of the band will be RFI free even when the servo system is ``on''. With the servo system `off', the RFI free frequency range in the spectrum when integration for 120 sec can be as high as 40 % or more. Many frequency ranges (especially with servo system `off') within this band can be used for astronomical observations. With the servo system `off', continuum observations in spectroscopic mode could use more than 40 % of any 40 MHz frequency band within the PF1 450 MHz band. 15 to 18 out of 27 galactic recombination lines are located at frequency ranges relatively free of RFI when the servo system is turned "off".